Using a newly developed method both for estimating relative levels of IgG1-class and IgG2-class antibodies of a give specificity in a serum, and for converting antisera or globulins to preparations containing only the IgG2 class or IgG1 class among the 7S antibodies of that specificity, it is planned: 1. to estimate the relative levels of alloantibodies of IgG1-class and IgG2-class, by determining the suppressive titers in mice immunized with allogeneic spleen cells both in the original serum and in samples brought to full expression of the IgG2 class of the antibody by treatment with anti IgG1. This will be done to see whether there is a relatively greater deficit of production of alloantibodies of IgG2 class which might account for the prolonged retention of allografts in aging animals. 2. in inbred mice bearing autochthonous tumors, to study the relation between the levels of anti-tumor antibodies of IgG1-class and IgG2-class produced, in order to see whether there is a special deficit of IgG2-class anti-tumor antibodies which might account for the greater susceptibility of aging mice to transplanted tumors.